The present invention relates to a plate-making method for treating a light-sensitive lithographic printing plate (an original plate for lithographic printing plate having a light-sensitive layer provided thereon; hereunder also referred to as “light-sensitive plate”) on which an interleaf paper is superposed using a plate-setter equipped with a mechanism for automatically removing the interleaf paper and an automatic developing machine as well as a plate-making system used in the foregoing plate-making method. The present invention also relates to a plate-making method for treating a light-sensitive lithographic printing plate on which an interleaf paper is superposed, and an original plate for making a dummy plate having an interleaf paper superimposed thereon using a plate-setter equipped with a mechanism for automatically removing the interleaf paper and an automatic developing machine as well as a plate-making system used in the foregoing plate-making method.
Paper called interleaf paper is in general superimposed on the plate surface of a light-sensitive lithographic printing plate in order to prevent the formation of any defect on the plate surface possibly encountered when transporting the same or accommodating a plate in a plate-setter in which CTP (computer to plate) is imagewise exposed to a laser beam and to prevent any adhesion of a plate to another one under high humidity conditions. In addition, there has conventionally been used a plate-making device provided with a means for automatically removing an interleaf paper from the plate surface and disposing the same (hereunder referred to as “automatic interleaf paper-removing mechanism”) during the plate-making process (see, for instance, Patent Document 1 listed below). In such a plate-setter provided with an automatic interleaf paper-removing mechanism, the interleaf paper is automatically removed from the plate surface. When the interleaf paper is not removed for some reason and the plate carrying the interleaf paper is exposed to a laser beam, however, the interleaf paper is often scorched or burned. In addition, when the light-sensitive plate is a photopolymerizable lithographic printing plate and the plate is fed to an automatic developing machine together with the interleaf paper, the interleaf paper is scorched and optionally burned and thus, this may become the origin of a fire in a pre-heating zone of the developer. More specifically, the first step in the automatic developing machine is one in which the light-sensitive plate is heated to a temperature of about 125° C. with a ceramic heater or hot air in the pre-heating zone and therefore, the temperature of the heater is increased even to 650° C. Moreover, if the interleaf paper is introduced into a pre-water-washing zone and/or a developing zone, the interleaf paper is broken into small pieces, dispersed into water or a developer and this may result in the clogging of the piping system of the developing machine or this may optionally make the use of the developer impossible. Further, when the light-sensitive plate is a thermally cross-linkable light-sensitive lithographic printing plate and the plate carrying an interleaf paper superposed thereon is fed to a heating oven, the interleaf paper is scorched, optionally burned and thus, this may become the origin of a fire therein since the plate is heated to a temperature of about 150° C.
Moreover, in case where in, for instance, color newspaper printing, the printing operation must be carried out by two-color or mono-color printing since the whole space is not subjected to the color printing, it is necessary to conduct such printing operations while fitting a printing plate free of any light-sensitive layer (dummy plate) onto the plate cylinder, which is not used. There has not yet been developed any dummy plate specially designed for CTP and therefore, a light-sensitive plate is developed without exposure to light to give a dummy plate or an original plate for making a dummy plate for a conventional PS plate is developed in an automatic developing machine generally used for treating the conventional PS plate. However, this suffers from problems such that the formation of a dummy plate by such a method is quite disadvantageous from the economical standpoint and that the practice of such method requires the use of a quite large space since two lines of plate-making systems should be provided. In this respect, when using an original plate for making a dummy plate specially designed for CTP, which comprises an aluminum plate provided thereon with a light-insensitive layer, it is required to use only one line of plate-processing system and this is quite advantageous from the economical standpoint. However, the interleaf paper superposed on such an original plate for making a dummy plate may suffer from problems similar to those discussed above.
In this respect, there has conventionally been reported the development of a device for detecting light-sensitive lithographic printing plates using a semiconductor laser beam in order to prevent any supply, into a plate-making device, of a plurality of light-sensitive lithographic printing plates, at a time, which are adhered to one another when they are stored in the superimposed conditions (see, for instance, the patent document 2 specified below). In this device, the side of printing plates fed to a plate-making device is irradiated with a laser beam to thus detect whether a specific printing plate fed thereto comprises at least two plates adhered to one another, or not on the basis of the time required for the side thereof to transverse the laser beam.    Patent Document 1: Japanese Un-Examined Patent Publication (hereunder referred to as “J.P. KOKAI”) Hei 10-323961    Patent Document 2: J.P. KOKAI Hei 6-87551